In the chemical industry, foodstuff industry and dyestuff industry it is frequently necessary to mill granulate or pulverulent materials in the wet state and/or to disperse them in a liquid. This is normally done in agitator mills comprising a mostly cylindrical grinding container in which coaxially with the container an agitator shaft is arranged. Agitating elements are mounted to the agitator shaft. The grinding container is partly filled with hard grinding bodies being resistant to wear to a large degree, whose size is chosen in dependency upon the desired result of the grinding operation. The grinding container has an inlet and an outlet for the material to be ground, the inlet being at the bottom and the outlet at the top in most cases. Upstream of the outlet, a separating device is generally provided allowing only the ground material to pass but retaining the grinding bodies.
In the operation of such agitator mills difficulties may arise in that the viscosity of the material to be ground increases strongly during the milling operation since the liquid with which the solid particles have been wetted outside the agitator mill is not capable of forming a sufficient liquid film around each particle even when the same are more or less comminuted, and consequently their overall surface area becomes increasingly larger. When the viscosity of the material to be ground in the grinding container increases as the distance from the inlet increases this entails an increase of the shearing forces acting upon the material to be ground as a consequence of the relative movement thereof in respect to the agitating elements and the grinding bodies. During the grinding operation the material to be ground therefore takes up the more mechanical work per time and volume unit the more material to be ground moves away from the inlet. An undesireable consequence of this increased work take-up is a heating of the material to be ground which increases also as the distance from the inlet increases. In order to prevent that the material to be ground is damaged by overheating or that even waste is made thereof it is mostly necessary to reduce the rotational speed of the agitator shaft and/or the density of the grinding bodies in the grinding container and to pass the material to be ground in circulation several times through the grinding container and to dilute the material to be ground with a liquid each time before it enters the grinding container.
From German Pat. No. 1,607,441 an agitator mill has been known in which the difficulties described hereinbefore are lessened in that the grinding space is subdivided into two or more grinding chambers containing grinding bodies of different sizes and being separated from one another by at least one processing chamber free from grinding bodies. A conduit opens into the or each processing chamber through which air or a neutral, non-oxidizing gas, or any other treating medium or an additive is to be supplied into the material to be ground. The difficulties described cannot be avoided altogether thereby, however, since the highest heating of the material to be ground occurs in one of the grinding chambers where the grinding bodies are present and since only there a diluting medium can be mixed rapidily and thoroughly with the material to be ground.